Calculating Grade 8 Shear Strength

What is the shear strength of Grade 8 bolts?

A common question that we get here at Portland Bolt is in regards to the shear strength of bolts. The shear strength is the value at which the lateral stress on a bolt or screw causes it to fail. It is an important factor to take into consideration when designing structural steel or timber connections. Shear strength is also an easy value to roughly calculate when the ultimate tensile strength is known.

Below is a transcript of a recent live chat that we had on our website addressing shear strength. The chat was fielded by our very own Dane McKinnon and serves as a good example of what kind of response you can expect when you ask us a question. We’re substituting a generic name below for the customer to protect their privacy.

Dane McKinnon:They are all the same tensile strength per square inch. The larger ones have a larger cross section of material, and so are therefore stronger. To calculate the tensile strength of a particular size, you would multiply 150,000 psi by the tensile stress area, found here: http://www.portlandbolt.com/technical/thread-pitch-chart/

Henry:Thank you

Henry is now off-line and may not reply. Currently in room: Dane McKinnon.

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21 comments

I was sent 1″ diameter bolts (8″ long) by a treehouse company. Don’t know the grade, but presume that treehouse companies must have the purpose in mind. Any rough estimate on a shear capacity for such? I want to be safe.

My car weighs 5000#. It has four cylindrical jacking points built into the uniframe … on each side of the car, one point behind each front wheel and one point just forward of each rear wheel.

When performing work on the car’s brakes, I commonly jack the car from axial center-line jacking points with a floor jack and insert a 10″ Grade 8 3/4″-10 bolt into each jacking point, then set the car atop 4 jack stands, each supporting a 10″ Grade 8 bolt.

Is there any realistic risk that one or more of the bolts might fail … bend or snap in two … under these facts?

@Arlen- The minimum shear strength of one 3/4-10 grade 8 bolt is approximately 30,060lbs, so we don’t see a scenario where your 5,000lb car should bend or break any of the bolts. Jack stands are typically rated from 3 to 12 tons, so we’d think that the jack stand would fail before the bolt does.

@Clayton – No, 1/8″ is too small to be covered under most SAE or ASTM fastener standards. The only standard I am aware of that covers small screws is ASTM A574 for socket head cap screws. In that case the tensile is 180ksi minimum and the proofload is 140ksi.